if its not much more, get the biggest fastest drive you can afford. it will make your G4 go a lot faster. i think hitachi has a 200 gig drive that's seriously fast.
hd's are the weakest point in current machines. If you can jack up the speed, it makes a lot of difference. Plus, OSX goes slower when the drive gets full. (i think barefeats did benchmarks on that awhile ago.)
zug